Electrical contact system



July 27, 1948. .w. F. ABBO'IT 2,445,315

ELECTRICAL CONTACT SYSTEM Filed Jan. 22, 1944 3 Sheets-Shoat 1 In uenlor Altorney July 27, 1948. w. F. ABBOTT 2,445,816

' ELECTRICAL CONTACT SYSTEM I Filed Jan. 22, 1944 3 Sheets-Sheet 2 H F/GZ TEMI? TIME y 1943' w. F. ABBOTT I ELECTRICAL CONTACT SYSTEM 3 Sheets-Sheet 3 Filed Jan. 22, 1944 In enter Allorney Patented July 27 1948 ELECTRICAL CONTACT SYSTEM Winfield Forbes Abbott, Rulslip, England, as-

slgnor to The Bristol Company, Waterbury, Conn., a corporation of Connecticut Application January 22, 1944, Serial No. 519,331

In Great Britain February 9, 1943 'This invention relates to electrical contact systems and in particular to highly sensitive relays such as contact making galvanometers and to their use for controlling and/or recording and/or indicating temperature, speed, acid concentration or any other physical effect which when changed I can be made to produce corresponding changes of electrical potential.

It is well known that thermo-electric instruments of both the potentiometric and direct deflection type are used for this purpose. In both cases a sensitive galvanometer of conventional I assume a condition of balance between the ten-' sion of its restraining spring and the torque of the coil and is then momentarily clamped in this position by the switch mechanism which senses the position of the pointer or needle and is set on or "off thereby. After a period the pointer is released and the cycle of operations repeats.

The disadvantages of this method lie in the difllculty of combining a highly sensitiv movement with the robustness necessary to withstand the continual clamping and disengaging operations and in the fact that a period of time must elapse before a change of condition can be altered by the switch. It is also a disadvantage to have machinery constantly in motion to perform the switching operation. When larger forces than those with which we are at present concerned are available it is known to bring a contact piece on the moving pointer into engagement with a fixed contact and so to switch on" and oil directly without auxiliary devices. In the case of highly sensitive galvanometers there is not sufficient power to overcome quickly the inevitable electrical resistance between the contacts when making contact, and delay is experienced in overcoming the tendency of the moving contact piece to cling when moving out of contact. For these reasons it has seldom been found expedient or consistent with accurate measurement or dependable performance to use highly sensitive galvanometers as direct contact-making devices.

Various expedients have been suggested for superimposing upon the normal force of galvanometers a force derived from a supplementary current, flowing'either in the galvanometer coil or in an auxiliary winding assembled upon the 12 Claims. (01. 236-69) galvanometer shaft; but experience has shown that, where the galvanometeris required to make contact) in either of two directionsof deflection, such difliculties are encountered as to establish serious limitations to the close settings of the contacts, with a consequent sacrifice of precision in order to secure satisfactory operation.

An object of the present invention is to provide improved means for employing a highly sensitive galvanometer system as a contact making relay. It is a further object to provide a system of "high and low contacts which maybe set very closely relatively to each other without producing a hunting action,

Another object is to provide a controlling device capable of producing corrective action to some extent proportional to the rate' of change of controlled condition and also proportional to the extent of departure ofthe condition from the desired point.

With the above objects in view a feature'of the present invention is the employment of two pairs of contacts, one contact of each pair being stationary and the other being movable, and the control applied to the twomovable contacts being action. The relative disposition of the stationary.

and movable contacts under a condition of mechanical balance, and with no current flowing in the galvanometer windings may be established to fulfill any one of three conditions: (1) the contacts may be set to lie in immediate mutual juxtaposition but not in actual electrical en agement; (2) the contacts may be set to lie normally some distance apart; or (3) the contacts or springs may be given such initial tension as to maintain themovable and stationarycontacts normally in positive electrical engagement.

The movable contacts above referred to are conveniently carried on the movable pointers or needles of two excessively damped moving coil galvanometers connected in parallel; it will be understood that each movable contact. by being associated with only one stationary contact, is free to move away therefrom to any desired extent.

According to a modification by way of simplification applicable where control to preserve a steady value is not necessary-but it is required only to prevent a certain value from being exceeded, as in the case of preventing rise of temperature beyond a certain degree, a single galvanometer with a single movable contact and a single stationary contact co-operating with a boosting resistance and relays may be employed as hereinafter explained.

For simplicity of further description but without implying limitation of the features of the invention to one particular industrial application, the invention will hereinafter be described with reference to temperature control in a furnace by means of a thermo-couple co-operating with a pair of moving coil galvanometers, a group of relays controlling the supply of heating medium to the furnace, and a potentiometer circuit so set that the voltage developed across the thermocouple by its exposure to the heat to be controlled becomes balanced against the potentiometer whenthe desired temperature is reached.

In order that the invention may be clearly understood and readily carried into effect I will now describe the same more fully with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a two-galvanometer system in accordance with the present invention applied to temperature control of a furnace.

Figures 2, 3 and 4, are temperature-time graphs to assist in the explanation, but it, is understood that these graphs are not to be regarded as drawn to any particular scale. These figures also show graphically the application of heat to the furnace. Figures 5, 6 and 7, are representations of the two needles and moving contacts of the two galvanometers relative to each other and to their respective stationary contacts.

Figure 8 represents in a manner similar to Figure 1 a modification by way of simplification applicable in certain circumstances.

Referring to Figure l, A is the furnace whose temperature is to be controlled by thermo-couple B. The elements of the thermo-couple are connected the one to a temperature setting arm C co-operating with a resistance C at one of the conjugate points of a potentiometer or bridge network D, and the other to the other conjugate point C of the network in series with a resistance R, and two circuits in parallel containing respectively two resistances 1, T and the two coils G G of the two galvanometers, The two resistances 1r are alternatively short circuited by contacts 1" and r as hereinafter explained.

The source of voltage used for balancing against the E. M. F. developed in the thermocouple B is shown at D connected across the other pair-of conjugate points of the potentiomete1.

The galvanometer coil G controls a pointer or needle carrying a contact g co-operatlng with a stationary contact g Similarly the galvanometer coil G controls a pointer or needle carrying a contact g co-operating with a stationary contact g The arrangement of the coils G G and their needles and contacts is such that upon a specified direction of current flow in the common circuit of said coils they will simultaneously defleet in such a sense as to tend to close the contacts controlled by one needle and to open those controlled by the other, and both needles are free to move to a considerable distance (determined by stops) away from their stationary contacts, but when one needle is moving away from its stationary contact the other is restrained by com- 4 ing up against its stationary contact and vice versa. Thus, when, the actual temperature is considerably below a desired temperature T the galvanometer G is steadily in the oif" position, and G is steadily in the on position, and when the actual temperature is considerably above the desired temperature the galvanometer G is steadily in the on position and the galvanometer G is steadily in the off position.

R R and R are relay coils and Ti, r T r r, and r are corresponding relay contacts. R 1" are respectively the coil and moving contacts of a main feed relaydevice controlling the circuit of the power source MM supplying the furnace through contacts 1 r (when closed) with heating current for the heating element A. Coil R also controls the short circuiting of the one or the other of resistances r or r by cooperation of the movable contacts 1 with the stationary contacts 1' and 1. H H and H are the terminals of a mid-tapped battery or equivalent source of unidirectional current.

In order to understand the working of the system consider starting up with the furnace in a col-d condition, and suppose that it is desired to raise its temperature to and maintain it approximately at a value represented by T, between limits designated as T and T in Figs. 2, 3 and 4. There may first be considered an example where the furnace has little thermal inertia and the thermocouple B a negligible lag of response so that the value I and T may substantially coincide, as indicated in Fig. 2. I will assume that the position of the needles and their contacts relative to the stationary contacts has been set in a state of mechanical balance as illustrated in Figure 5, when no current passes through either coil; both moving contacts are on the point of engagement with their respective stationary contacts but are not restrained thereby.

The arm 0 of the potentiometer is set at the desired temperature T and there will be a condition of unbalance as between the thermo-couple and the potentiometer. This will send a current through coils G G in a direction to drive moving contact g away from g, and g will be forced toward and restrained by g but no electrical circuit can be closed by this action while the relay coil R is de-energized. The relays and their circuits will be as shown in Figure 1, i. e. the contacts of relay coil R will be in their normally open position, the contacts of coil R in their normally closed position, and the contacts of coil R in a position to close the circuit of coil R and thereby keeping the contacts r 1 .closed. Since contacts 1 1'" are in series with the furnace heating element A and the main supply M M the furnace will be supplied with heating current. The furnace will thus become increasingly hot and increase the temperature of the thermo-couple, and the state of unbalance between the thermocouple and the potentiometer will decrease until it reaches temperature T at which point no current will pass through the galvanometer coils G G Under this condition the galvanometer needles will tend to assume the position of mechanical balance illustrated in Figure 5 and contact will be made between contacts g and g. A circuit is' thus completed from terminal H through resistance R, contacts 1', g, 9 and r and the relay coil R. to terminal H The galvanometer action is however so delicate that the, resistance at contacts 99 is too great to permit a suflicient flow of current to enable relay coil R to close its contacts H. The small initial current r which flows upon engagement of contacts 00 will develop a potential across the resistor R. and this potential, being applied through the thermocouple B and the network D to the galvanometer G and being of the same polarity as that developed by said thermocouple, will .serve to increase the pressure between the contacts ga the lowered resistance at gg permits more current to flow, thus further increasing the pressure at 99 and this process continues until the increased current is sufficient to cause the relay coil R to close its contacts 1 Meanwhile, the relatively high current that has energized thegalvanometer coil G to aid in closing gg has also energized the galvanometer coil G, but to a less degree owing to the efiect of series resistance T and has sent contact g away from contact o. By the closing of the contacts 1 the coil R is energized by current from terminal H throughcontacts r and r to terminal H The energizing of coil It causes its armature to rise and produce de-energizing of 1 the coil Rwhioh allows the armature thereof to drop and open contacts 1 1 thereby removing the heating source from the furnace. The rise of the armature of coil It opens the circuit of coil R at contacts '1' so that contacts 1' separate but the coil R is maintained energised by the closing of contacts 1'. It having been assumed that the furnace has little thermal inertia and that the thermo-couple is very sensitive, it may be assumed further that upon the removal of the heating source no further temperature rise will take place and that, as heat is lost to the surrounding air, the temperature will tend to drop. There will, however, be an interval of time, while the trend is changing from one direction to the other, during which the temperature may be considered to be stationary, this point coinciding substantially with T. As stated earlier, while the temperature is at T no current is passing through the galvanometer coils G G, therefore coil G will move from the position to which it was carried by the action of the superimposed current, to the position of mechanical balance but will do so relatively slowly because of the high degree of damping which is a feature of the galvanometer movement employed. If the time taken is great enough to allow the temperature oi. the furnace to be reduced below T this will cause a state of unbalance between the potentiometer and the thermo-couple and therefore cause a current to flow through coils (l G in .a sense to hasten the movement of contact g toward. contact a. When contact is made at 9 9 a circuit is completed from terminal H through relay coil R, contacts r, a, 9 and r, and resistance R to terminal H Through the action of the boosting resistance R, extra current is superimposed on the coils G G as previously described, causing 9 9 to close firmly and energize relay coil R thus opening the contacts 1 This breaks the circuits to the coil R and allows the armature I to fall and close contacts 1 and. r and open contacts 1 and r. The closing of contacts 1 energizes the coil R which closes contacts 1 1', thereby again supplying heat to the furnace. Meanwhile contact 9 had responded to the superimposed current which had secured flrm engagement of contacts Mg and had been displaced to a ditsance from 9 It is now moved slowly toward 9 under the influence of the galvanometer control springs, and, as the temperature tends to rise above T, also underthe influence ofthe potentiometer. when contact 9 reaches contact a a circuit is established as previously described and the cycle repeats in the same way as above described. Under the influence of the coils G and G alternately, the heat is alternately applied and removed for intervals of suflicient duration to counterbalance the heat losses and so maintain the temperature '1 substantially constant as shown by the temperature time graph in Figure 2 on which the application and removal of heat are also shown by the line H. i

I will now consider the application of this invention to a furnace such as that in which the heating elements or burners are placed in a compartment known as a muilie, separated from the working portion by walls of refractory brick or the like. Such a furnace has considerable thermal inertia due to the capacity of the walls to store and give up heat. I will also assume that the thermo-couple instead of being very sensitive is protected by a metallic or refractory sheath which tends to delay its response. To make the system more suitable for this condition, the position of the stationary contacts and g relative to the position 01 mechanical balance of the moving ones a and 9 is such as to restrain 90 as illustrated in Figure 6. If the contacts 90 were unrestrainedby the stationary contacts 9 and 9 the contact a would assume theposition g", and the contact g would assume the position g. That is, whenv the galvanometer coils G G" are in a state of mechanical balance, their movable contacts y and 0 both tend to press against their respective stationary contacts-g andg by the action of their springs. The performance of such an arrangement may be understood by reference to Fig. 3, in which T represents the temperature at which a state of balance exists between the thermo-couple. and the potentiometer, at which point no current will flow through the galvanometer coils (PG and 'I' may be considered as the temperature at which such a state of unbalance would exist as to cause suflicient current to flow through the coils G G" to balance exactly the action of the restraining spring of coil G and would bring contacts: to the point oi leaving g '1 may be considered the temperature at which a state of unbalance would exist which would cause current to flow through the coils G G in a direction opposite to that established at T and which would balance exactly the action of the restraining springs of coil G and cause contact 9 to be on the point of leaving 9 The difierence between temperatures TT may be considered very small relative to the temperature T.

Starting with the furnace ina cold condition, the arm C will be moved on the resistance C to the position T and while the furnace temperature is steadily below T the position of the various relays will be as shown in Figure 1 and the contact y will be away from contact 9 while contact o will be pressed against contact 9. Heat will be applied to the furnace through the closed flow of current through coil G caused by an unbalance between the thermo-couple and' the contacts r 1 When the temperature has risen to T the condition of unbalance between the thermo-couple and the potentiometer will cause a suflicient flow of current through coils Gr G to bring the contact a into contact with the contact g Extra current is then superimposed as previously' explained to cause firm closure of go and the coil R becomes energized. This in turn energizes coil R opening contacts 1 which deenergizes coll R and opens the heating circuit at rr". Meanwhile the superimposed current which caused firm closure of the contacts ga has acted 7 on coil G to send contact g away from contact a but against considerable spring tension. The actual displacement of contact g is therefore relatively small and the restoring force relatively large so that in spite of the excessive damping of themovement, the contact g is returned to bear against contact g in a comparatively short time. While it is returning, heat is still being imparted to the working portion of the furnace from the muflie although the supply of heat in the muiile has been removed. As the result of this action the temperature-time curve just above T will continue at practically the same rate as just under this point. As contact g makes contact with 9 heat willagain be applied to the furnace by a sequence of events previously described, and the contact y will be displaced from the contact g by the superimposed current. It will assume that in the short time during which the heat has been oft, the temperature is not greatly above T 50 that the force of the galvanometer control spring to restore the contact g to contact with g will not be very great and the time taken will be long relative to that taken by the contact a? to return to g in its last described operation. During the time it is returning the temperature is continuing to rise. When the heat is removed for the second time, and the contact g is displaced from g it will tend to move farther and the returning force will be less through the effect of the higher temperature causing less current to flow through the coil G. Therefore the second "01? period will be longer than the first, and when heating is again applied it will remain on" for a. shorter time than during the previous period. This process will continue until, at temperature T, thetforces acting on the two galvanometer coils being equal, it may be considered that the off time as compared to "on" time will be substantially the same. As the temperature continues to rise above T, the on time will be progressively less in relation to the o time until, as T is passed, the contact y will remain away from the contact 91 and will cause the heat to be continuously off until the temperature has fallen to T at which point heat is applied for relatively short intervals but increasing in duration as the temperature falls. The eflect of the action just described'is to produce a flattening of the temperature-time curve as illustrated in Figure 3 by taking account of the approach of the temperature to the desired value T and making allowance for the continued temperature trend upward and downward after the heating source has been removed and applied. It is to be observed that in Figure 3 the line H illustrates when heat is on or ch in relation to the trend of .the temperature-time raph.

Consideration may now be given to a furnace having relatively little thermal inertia and in which the temperature does not need to be controlled within fine limits and frequent applications and removals of heat are not wanted.- Figure '7 illustrates the position of the stationary contacts g and g relative to the moving contacts a and 9 when the galvanometer coils G G are flows in the coils G G thus allowing gg to take the position of mechanical balance in the positions illustrated. T may be considered the temperature which will produce an unbalanced condition just sufflcient to place contact 9 on the point of engagement with contact 9 and T may be considered the temperature which will produce just sufficient unbalance to place con-. tact g on the point of engagement with contact g Referring momentarily to Figure 1, the arm C will be set at T and, starting with the furnace in a cold condition, the relays will be in theposition illustrated. While the temperature is below T the contact 9 will be steadily away from contact g and below the temperature T the contact g will be steadily in engagement with contact 9 Upon the temperature reaching T the contact g makes contact with contact 9 and through the sequence of events previously described heating is removed from the furnace. The heating remains steadily o until the temperature has dropped to T at which point the contact g engages with contact g and thereby brings it on= again. This cycle is repeated continuously as illustrated on the graph in Figure 4, which also shows by means of the line H the application and removal of the heating means.

Figure 8 as already indicated illustrates an application of the invention which does not require all the refinements previously described. A is a furnace from which it is desired to remove the heating supply when the temperature exceeds a safe limit but which it is not intended to regulate as to temperature. Assume that the temperature at which it is desired to limit the furnace is'T degrees. B is a thermo-couple located within the furnace. One side of the thermo-couple is connected to one side of a galvanometer coil G through resistances S andR, and the other side of the thermo-couple is connected to the opposite side of the galvanometer coil..

This galvanometer has a single stationary contact g for co-operation with a contact g on the needle. The initial mechanical setting of the galvanometer is such that the contacts are normally together, but not at appreciable contact pressure. Connected in series with resistance S is a milliameter A a variable resistor V and a battery B. At the junction of the resistances S and R is connected one side of a D. C. source of E. M. F. H the other side H being connected through a relay coil R and contacts 1 to the fixed contact 9 R and R are relay coils, and r 1', r and r are relay contacts. E represents the furnace heating element, and F and F and M M are electrical mains; B is a manually operated push button.

A portion of the current from the battery B when not opposed by current from the thermocouple (which is the case when the thermo-couple is not generating current), passes through resistance S and the galvanometer G and shifts the contact g away from the contact g When the thermo-couple is acting, the current from it is made to oppose that from the battery B and by adjusting the resistance V the two currents can be made to neutralise one another, and thereby enable contacts gg to come together. The higher the value of the current indicated by A the higher will be the temperature at which the coil G will reach balance and so cause the contact 9 to touch g and the scale of the milliammeter A" may be marked in terms of temperature. If I set resistance V to a value that will cause the milliammeter A to indicate temperature T, and the furnace A is in a cold condition, the relays will be in the position as shown in Figure 8. An unbalanced current will flow said sense upon a decrease of said current, and

to swing freely in the opposite direction upon an increase of said current, and relay means subject to actuation under the joint influence of said circuit controlling means, for controlling said condition.

3. In an electrical system for limiting departure of the magnitude of a condition from a preg. This will complete a circuit from terminal H through resistance R, contacts g, g and r and relay coil R to terminal H As previously described, the effect of closing this circuit secures firm closure of the contacts gg and current passes through the coil R which closes contacts r and complete a circuit from terminal F through contacts 1 coil 1'1. anda switch B to terminal F When the coil R is energized contacts r and r are opened, and contact 1' is closed. The opening of contacts 1* removes the heat supply from the furnace and contacts 1' break the circuit to the coil R and contacts r complete a circuit for keeping the coil R energized. The heating supply has thus been cut off and cannot be returned untilthe attendant has broken the circuit to the coil R. by opening the switch B.

If it is desired merely to record a changing condition or value such as a furnace temperature instead of control it, this may be accomplished by converting the spindle of the arm G into the shaft of a small reversible electric motor and by driving a recording pen by gearing from the shaft, the power circuits of the motor being controlled by the galvanometers and relays asalready described and the armature being driven in one or other direction whenever a condition of balance in the Wh'eatstone bridge arrangement does not prevail.

What I claim is:

1. In an electrical system for limiting departure of the magnitude of a condition from a predetermined value, the combination of a pair of galvanometer elements interconnected in a common circuit, means for affecting the flow of an electric current in said circuit in response to variations in said condition, one of said galvanometer elements being provided with circuit controlling means and adapted to actuate the same in a selected sense only upon an increase of current in said circuit and the other of said galvanometer elements being also provided with circuit controlling means and adapted to actuate the same in said sense only upon a decrease of said current, and relay means operable upon the joint influence of said circuit controlling means, for controlling said condition. i

2. In an electrical system for limiting departure of the magnitude of a condition from a predetermined value, the combination of a pair of defiectable galvanometer elements interconnected in a common circuit, means for afiecting the fiow of an electric current in said circuit in response to variations in said condition, one of said galvanometer elements being provided with circuit controlling means and adapted to be deflected in a direction to actuate the same in a selected sense upon an increase of current in said circuit and to swing freely in the opposite direction upon a decrease of said current, and the other of said galvanometerelements being similarly provided with circuit controlling means and adapted to be deflected in a direction to actuate the same in determined value, the combination 01. an electric circuit and means for affecting the flow of current in the same in response to variations in said condition, a pair of galvanometers connected in said circuit to carry in each of'their actuating windings a portion of said current, means for rendering unequal the currents in said respective windings whereby to cause the current in one of said windings to be greater upon response of said galvanometers to a current change representing an increase in the magnitude of said condition, and for causing th'e current in the other 01' said windings to be greater upon response of said galvanometers to current change representing a decrease in said condition, circuit-controlling elements actuated by said galvanometers, and relay means subject to actuation under the joint influence of said circuit controlling elements, for controlling said condition.

4. In an electrical system for limiting departure of the magnitude of a condition from a predetermined value, the combination of a pair of galvanometers each having only a single stationary contact and a movable contact, means responsive to said condition and controlling said galvanometers for causing said movable contacts to be similarly disposed in relation to their stationary contacts when the magnitude of said condition has a given relation to said predetermined value and for causing the disposition of said movable contacts with respect to said stationary contacts to be dissimilar when said magnitude departs from said relation, and control means for said condition comprising a pair of relays each having an energizing circuit governed by a corresponding pair of said galvanometer contacts.

5. A system as defined by claim 4 where the movable contacts are normally out of engagement with the associated stationary contacts when said condition has a given relation to said value, the spacing between the contacts tending to decrease toward a condition of mutual engagement, and that between the contacts of the other tending to increase, with an increase of said magnitude above said value, the sense of said spacing changes being reversed upon the decrease of said magnitude below said value.

6. In an electrical system for limiting departure of the magnitude of a condition from a predetermined value, the combination of an electric circuit, means for producing therein a flow of current in a direction and of an intensity representative of said departure, a pair of deflectable galvanometerelements both connected in said cirsaid circuit in response to variations in said or one galvanometer condition, one oi said galvanometer elements becontrolling means and adapted to be deflected in a direction to actuate the same in said sense upon a decrease of said current. and to swing unobstructedly in the opposite direction upon an increase of said current, relay means subject to actuation under the joint influence or said circuit controlling means, and condition aflecting means commanded by said relay means.

8. In an electrical system for limiting departure of the magnitude of .a condition from a predetermined value, the combination of an electric circult, a source of potential in said circuit sensitive to changes in said condition and adapted to introduce into said circuit a, potential representative of said magnitude, a further source of potential included in said circuit, and means for adjusting said last-named source at will to produce a potential representative of said predetermined value, a, pair of galvanometer elements interconnected in said circuit and jointly responsive to the current flowing therein due to the algebraic sum of said potentials, contact members actuated independently by said galvanometer elements, means subject to said contact members for modifying the distribution of current between said galvanometer elements, together with relay means subject to the joint influences of said contact members for controlling said condition.

9. In an electrical system for actuating temperature controllingmeans to limit the departure of a temperature from a predetermined value, the

a thermocouple exposed to said temperature to produce an electromotive force corresponding thereto, a bridge network including a source of electromotive force and adjustable whereby a predetermined part of the same may be introduced into said circuit, a first galvanometer element having a contact member and connected in said circuit to deflect in a sense to close said contact member in response to an increase of current in said circuit, and to swing unobstructedly in the opposite sense upon a decrease of said current, a second galvanometer element having a contact member and connected in said circuit to deflect in a, sense to close said contact element in response to a decrease of current in said circuit and to swing unobstructedly in the opposite sense upon an increase of said current, a first relay, a second relay, means for rendering said first relay subject to the contact memberof said first galvanometer element and said second relay to that of said second galvanometer element, a third relay subject to said first relay and adapted when energized to render said first relay insensitiveto said first galvanometer contact member and to render said second relay sensitive to said second galvanometer contact member and a fourth relay subject to said third relay, adapted to actuate said controlling means.

10. In an electrical system for actuating temperature controlling means to limit the departure of a temperature from a predetermined value, the combination of a first electrical circuit including a thermocouple exposed to said temperature to produce an electromotive force corresponding electromotive force and adjustable whereby a predetermined part of the same may be introduced into said circuit, a first galvanometer element having a contact member and connected in said circuit to deflect in a sense to close said contact member in response to an increase of current.

in said circuit. and to swing unobstructedly in the opposite sense upon a, decrease of said current, a second galvanometer element having a contact member and connected in said circuit to deflect in a sense to close said contactelement in response to a decrease oi current in said circuit and to swing unobstructedly in the'opposite sense upon an increase of said current, a first relay, 9, second relay, means for rendering said first relay subject to the contact member of said first galvanometer element and said second relay to that of said second galvanometer element, a third relay subject to said first relay and adapted when energized to render said first relay insensitive to said first galvanometer contact member and to render said second relay sensitive to said second galvanometer contact member and a fourth relay subject to said third relay, adapted to actuate said controlling means, and also having contacts adapted to'modify the intensity of response of said galvanometers to current flowing therein.

l1.In an electrical system for limiting departure oi the magnitude of a condition from a predetermined value, the combination of a pair of galvanometers each having a stationary contact and a movable contact, means responsive to said condition and controlling said galvanometers for causing said movable contacts to be similarly disposed in relation to their stationary contacts when the magnitude of said condition has a given relation to said predetermined value and for causing the disposition of said movable contacts with respect to said stationary contacts to' be dissimilarwhen said magnitude departs from said relation, and control means for said condition comprising a pair of relays each having an energizing circuit governed by a corresponding pair of said galvanometer contacts, said movable contacts barely touching their associated stationary contacts without pressing against them when the magnitude of said condition has a given relation to said value.

12.-In an electrical system for limiting departureof the magnitude of a, condition from a predetermined value, the combination of a pair of galvanometers each having a stationary contact and a movable contact, means responsive to said 5 condition and controlling said galvanometers for causing said movable contacts to be similarly disposed in relation to their stationary contacts when the magnitude of said condition hasa given relation to said predetermined value and for causing the disposition of said movable contacts with respect to said stationary contacts to be disesimilar when said'magnitude departs from said relation, and control means for said condition comprising a pair of relays each having an energizing circuit governed by a corresponding pair of said galvanometer contacts, said movable con- I tacts being normally strained into engagement with the associated stationary contacts when said condition has a given relation to said value, the force or said engagement between the contacts of one galvanometer tending to increase, and that between the contacts of the other tending toward separation, with an increase of said magnitude thereto, a bridge network including a source of above said value, the sense of said force changes 13 14 being reversed upon the decrease of said masmm we TENT tude below said value. STA PA 8 wmrmm FORBES ABBOTT. Num r N me D Y 1,512,008 Otis Oct. 14,1924 REFERENCES CITED 1,924,459 Ryder M1529, 1933 The following references are of record in the file of this patent: 

